As the overall dimensions of semiconductor devices continue to shrink, the demand is ever increasing for devices having high charge storage capacity. The need for high charge storage capability is a result of the reduction in the chip area available for individual components as circuits are scaled to smaller dimensions. As the surface area of a component, such as a capacitor is reduced, a corresponding reduction in charge storage capability occurs. The smaller surface area available for components such as transistors, capacitors, and the like, coupled with the requirement to maintain high charge storage levels has motivated research for new materials from which to construct the components. One group of promising new dielectric materials is the family of PZT ceramic dielectrics. The PZT dielectrics are ferroelectric compounds of lead, zirconium and titanium oxides; hence the acronym "PZT."
Ferroelectric compounds are capable of being polarized by an applied voltage and retaining the polarization after the applied voltage has been removed. Responding to an applied voltage, the ferroelectric material assumes one of two remanent polarization states after the applied voltage is withdrawn. The ability to polarize a PZT ferroelectric and to design circuitry to detect the polarization state, makes the ferroelectric a desirable compound from which to construct non-volatile memory devices, such as non-volatile random access memory devices, and the like.
Thin films of PZT ferroelectric materials are commonly formed on semiconductor substrates using sol-gel process technology. Sol-gel processing provides a means of forming a thin PZT layer on a semiconductor substrate using existing spin-coating equipment. See for example, U.S. Pat. No. 4,946,710 to, W. D. Miller, et al., entitled, "Method for Preparing PLZT, PZT and PLT Sol-Gels and Fabricating Ferroelectric Thin Films", and U.S. Pat. No. 4,963,390 to R. A. Lipeles, et al., entitled, "Metallo-Organic Solution Deposition (MOSD) Of Transparent Crystalline Ferroelectric Films". Typically, a solution containing the desired metal elements is prepared by dissolving organometallic precursors in different organic solvents, then, mixing the various solutions together. After mixing, the solution is hydrolyzed to form a sol-gel solution. Following hydrolysis, the sol-gel solution is dispensed onto a semiconductor substrate. The substrate is held stationary, or is slowly spinning until the sol-gel solution is dispensed. Then, the spin speed is rapidly increased to drive off the solvents, and to leave a viscous sol-gel layer on the substrate surface. The amorphous sol-gel layer is then baked and sintered to form a perovskite crystalline compound having the general formula ABO.sub.3. In the PZT film, the A site is occupied by lead, and the B site is occupied by zirconium and titanium. In addition, a dopant such as lanthanum can be incorporated into the perovskite crystal in the A site location. The film is then commonly referred to as "PLZT."
While sol-gel technology has been widely used for the formation of PZT and PLZT thin-films, improvement the in sol-gel process is needed. In many cases, the PZT films do not exhibit optimum ferroelectric properties, such as high charge storage ability needed in a DRAM, nor the maximized remanent polarization necessary to meet non-volatile memory requirements. The non-optimum ferroelectric performance is often associated with an inability to completely crystallize an amorphous PZT material into the desired perovskite crystalline phase. Additionally, the ferroelectric film compositions vary from one process run to the next leading to non-reproducible film properties. An improved sol-gel process was therefore needed to provide a high quality, crystalline PZT and doped PZT films, wherein the amorphous sol-gel material is substantially converted to the perovskite crystalline phase.